As an optical transmission system has been advanced and become popular in recent years, multi channeling based on the WDM (Wavelength Division Multiplexing) system is being developed to increase transmission capacity of the system.
Beside this multi channeling system, in order to increase transmission capacity, there is a way of increasing a bit rate of optical pulses on each channel. 10 Gb/s has been now installed and recently 40 Gb/s is expected to be installed for the next generation.
In such a high-bit-rate optical transmission path, there are some factors for deterioration of transmission quality.
One of them is PMD (Polarization Mode Dispersion). This is a phenomenon such that in transmitted optical pulses, orthogonal polarization modes which should in theory be degenerating are separated to increase pulse width due to birefringence that occurs randomly in an optical fiber of optical pulse transmission path. Optical pulses with such a phenomenon can not serve as a right optical signal.
Accordingly, a study has been made to reduce this PMD in a recent optical fiber. However, this is at most 0.25 ps/km1/2. If such an optical fiber is used at the bit rate of 40 Gb/s, the distance which allows optical transmission is about 100 km at most, and optical transmission can not be realized at the distance more than 100 km.
In addition, PMD of an optical fiber installed in previous years is around 1 ps/km1/2 and therefore, when the bit rate is 10 Gb/s, possible transmission distance is about 170 km while when the bit rate is 40 Gb/s, possible transmission distance is decreased to be only 10 km.
Thus, in an optical transmission system with use of a previously installed optical fiber, if the bit rate is increased to be 10 Gb/s or if a new optical fiber is installed for the next generation and used at more than 40 Gb/s, PMD affects optical transmission significantly, which results in difficulty to construct a high-bit-rate practical optical transmission system.
For this reason, a PMD emulator is used to estimate a PMD characteristic in an optical transmission system which performs transmission at a high bit rate, and to compensate PMD which occurs in the optical transmission system.
Here, in order to evaluate PMD and the like of the optical transmission system, a conventional PMD emulator used to emulate PMD characteristics of an actually used SMF (Single Mode Fiber) is described. If PMD is estimated with use of an actual SMF in an experimental laboratory, as the experimental laboratory is an environment more stable than the place where the SMF is actually installed, more time will be required. Accordingly, it is effective to use a PMD emulator to perform PMD estimation.
An SMF used in this example is tens through hundreds of kilometer long and PMD is zero through tens ps approximately and two or more PMD are included. In this emulator, first-order PMD (DGD: Differential Group Delay) and second-order PMD (SOPMD) are handled.
A schematic view of the PMD emulator of this example is shown in FIG. 18. This PMD emulator is modeled by coupling plural (for example 100) DGD sections 1611 through 161n. In this example, a birefringent portion is used in a DGD section. The more DGD sections are used, the closer they are to the actual SMF characteristic. Therefore, in order to obtain PMD characteristic close to that occurring in an actual SMF, an extremely large-scale and expensive PMD emulator is required.
In order to obtain PMD characteristics of an SMF, DGD sections 1611 through 161n are rotated. The rotation speed is different between DGD sections however, each rotation speed is fixed and the speed is not controlled.
FIG. 19 is a graph showing DGD and SOPMD characteristics obtained by this PMS emulator. The horizontal axis of the graph indicates a wavelength (nm) and the vertical axis indicates DGD (ps) and SOPMD (ps2). FIG. 20A shows DGD distribution and FIG. 20B shows SOPMD distribution. Correlation between DGD and SOPMD at each wavelength is proved to be positive as shown in FIG. 21.
However, in order to obtain this PMD characteristic, a large-scale PMD emulator made up of a large number of DGD sections is used to be in operation for a long time.
Next, a typical example of PMD emulator in U.S. patent publication NO. 2002/080467 is descried specifically (see U.S. 2002/0080467, for example). FIG. 17 shows a schematic configuration of this device. Light to be measured is input into this device and then an accurate PMD can be obtained.
This PMD emulator 100 includes an input-side optical fiber 101 and an output-side optical fiber 102, birefringent portions 104, 106 and 108 which are composed of DGD portions 122, 132 and 142 and phase shift portions 124, 134 and 144, respectively and polarization mode mixing portions 110, 112 and 114. The birefringent portions 104, 106 and 108 and polarization mode mixing portions 110, 112 and 114 are connected by turn to make up plural stages (one birefringent portion and one polarization mode mixing portion consist in one stage). In FIG. 17, N stages made up of N birefringent portions and N polarization mode mixing portions are shown. The phase shift portions 124, 134 and 144 are provided with controllers 126, 136 and 146 for controlling a phase shift amount while the polarization mode mixing portions are provided with controllers 116, 118 and 120 for controlling polarization rotational direction.
Another conventional example is described below.
PMD generated in a transmission path installed previously is distributed in the temporal direction and in the frequency direction. These distributions are in accordance with theoretically shown probability density distribution, DGD (first-order PMD) is given a Maxwell's distribution and SOPMD (second-order PMD) is given a corresponding probability density function (see OPTICAL FIBER TELECOMMUNICATIONS, VOLUME IVB, Chapter 5 “Polarization-Mode Dispersion”). When transmission performance is tested, the distribution in the temporal direction is impotent, and used as a PMD emulator for simulating PMD on such an actual transmission path is a PMD emulator configured of multi-stage DGD sections of polarization maintaining fiber or a birefringent crystal, a variable polarization rotator being arranged between every two of the DGD sections and rotates them at randomly set rotation angle (see Proc.PFC02, paper ThA3, pp 374–375, 2002).
In the conventional PMD emulator disclosed in the U.S. publication No. 2002/0080467, used as a polarization mode mixing portion is YVO4 or LiNbO3. Since they utilize electro-optic effect, a large-scale device such as a piezoelectric element is required, which presents a problem of large power consumption. YVO4 and LiNbO3 also present a problem of large insertion loss.
Further in the conventional PMD emulator, in order to make the PMD temporal direction sufficiently close to theoretical probability density function, it is necessary to increase stages of DGD sections. Since in such a device, a large number of controlling portions are needed in accordance with the number of DGD sections, there occurs a problem that the device is complicated and expensive. In order to evaluate the optical system, it is necessary to obtain PMD characteristics by operating the device of many components for a long time.
Furthermore, in the conventional PMD emulator, in order to obtain statistic PMD characteristics (distribution), it is necessary to operate the PMD emulator for a long time until all data PMD values are obtained, and it is impossible to obtain only desired PMD values at some time.
Furthermore, there is a problem that in order to change an average DGD value, it is necessary to change DGD itself of each DGD section.
Furthermore, in the conventional PMD emulator, it is impossible to obtain PMD characteristic by fixing angle and not by changing connection angle.
Accordingly, the present invention was carried out in view of the problems of the related arts. An object of the present invention is to provide a PMD emulator which operates stably at low power consumption, which is low in insertion loss, which is configured of less components, which does not need a complex and expensive device, which generate a desired DGD value at some time, which allows an average DGD to be changed without changing DGD values of respective sections and which can obtain PMD characteristics without changing a connection angle.